Microwave heating of foods in a microwave oven differs significantly from conventional heating in a conventional oven. Conventional heating involves surface heating of the food by energy transfer from a hot oven atmosphere. In contrast, microwave heating involves the absorption of microwaves which may penetrate significantly below the surface of the food. In a microwave oven, the oven atmosphere will be at a relatively low temperature. Therefore, surface heating of foods in a microwave oven can be problematical.
A susceptor is a microwave responsive heating device that is used in a microwave oven for purposes such as crispening the surface of a food product or for browning. When the susceptor is exposed to microwave energy, the susceptor gets hot, and in turn heats the surface of the food product.
Conventional susceptors have a thin layer of polyester, used as a substrate, upon which is deposited a thin metallized film. For example, U.S. Pat. No. 4,641,005, issued to Seiferth, discloses a conventional metallized polyester film-type susceptor which is bonded to a sheet of paper. Herein, the word "substrate" is used to refer to the material on which the metal layer is directly deposited, e.g., during vacuum evaporation, sputtering, or the like. A biaxially oriented polyester film is the substrate used in typical conventional susceptors. Conventional metallized polyester film cannot, however, be heated by itself or with many food items in a microwave oven without undergoing severe structural changes: the polyester film, initially a flat sheet, may soften, shrivel, shrink, and eventually may melt during microwave heating. Typical polyester melts at approximately 220-260.degree. C.
Conventional polyester film has been thought to be necessary as a substrate in order to provide a suitable surface upon which a metal film may be effectively deposited.
In order to provide some stability to the shape of the susceptor, a metallized layer of polyester is typically bonded to a sheet of paper or paperboard. Usually, the thin film of metal is positioned at the adhesive interface between the layer of polyester and the sheet of paper.
During heating, it has been observed that metallized polyester will tend to break up during heating, even when the metallized polyester is adhesively bonded to a sheet of paper. Such breakup of the metallized polyester layer reduces the responsiveness of the susceptor to microwave heating. It has been observed that some areas of a conventional susceptor may initially heat substantially when exposed to microwave radiation, and then the heating effects of microwave radiation will appear to reduce. The responsiveness of those areas of the susceptor to microwave radiation decreases significantly as a result of breakup.
In the past, effective crispening and browning of a food surface using a conventional susceptor has been impeded because the metallized polyester layer presents a moisture impermeable food contact surface which inhibits the release of steam. Many foods release grease and water during heating. Trapped steam, water and fat between the food surface and the substantially moisture impermeable metallized polyester susceptor surface has an adverse effect upon crispening of the food surface.
Conventional susceptors are relatively costly to produce due to the multiple steps involved. First, a polyester layer is coated with a thin film of metal. Then this metallized polyester sheet is adhesively bonded to paper or paperboard. In some cases, this composite structure is further laminated to a final package.
U.S. Pat. No. 4,735,513, issued to Watkins et al., discloses an attempt to use backing sheets in addition to a coated susceptor substrate in order to maintain the structural integrity of the susceptor. U.S. Pat. No. 4,267,420, issued to Brastad, discloses a flexible susceptor film which includes a thin metal film on a dielectric substrate such as thin polyester. This thin structure may then be supported by more rigid dielectric material such as paperboard. U.S. Pat. No. 4,705,929, issued to Atkinson, discloses a rigid microwave tray and method for producing such a tray. A microwave interactive layer of material is provided on the upper face of the tray. None of these patents disclose a metallized layer deposited directly on a paper substrate.
It will be apparent from the above discussion that prior attempts to achieve a cost-effective metallized susceptor have not been altogether satisfactory.